Y Le*, Q He, J Lee, D Song, Johns Hopkins University, Baltimore, MD

Purpose: To investigate a novel method of characterizing prostate volume and shape changes after permanent prostate brachytherapy (PPB) by using the implanted seeds as a surrogate.

Methods: In twenty-one patients undergoing PPB, multiple 2D C-arm based fluoroscopy (CF) were taken intraoperatively immediately after seed implantation (day 0) and CT scan was performed the next day (day 1). The 3D coordinates of seed locations for each patient were reconstructed from both CF and CT, respectively. The volume of seed cloud was calculated as the volume of its convex hull. The CT and CF seed clouds were registered using iterative closest point (ICP) algorithm. The boundary seed shifts in each dimension between CF and CT seeds were calculated and student's t-test was used to test if the seeds shifts were significantly different from zero.

Results: Twelve patients show average of 7%±5% volume increase between day one CT seeds and day 0 CF seeds and nine patients show 9%±5% volume decrease. A positive correlation is found between seed-calculated volume and intraoperative preplanning ultrasound volume. For patients with volume increase, there is significant expansion in both z dimension (SI) with p<0.001. For patients with volume decrease, there is shrinkage in both x dimension (LR) with p<0.05. An average 23±8 degree pitch angle around x (LR) axis was observed between two seed clouds, corresponding to the change of patient position from lithotomy position during the implant to supine position during CT.

Conclusions: A method to use implanted seeds to determine prostate volume changes is demonstrated. The prostate volume can increase or decrease between end of implantation and day one CT, indicating edema either reaches its maximum intraoperatively, or still continues development after implantation. The ability to characterize immediate postoperative prostate volume and shape changes allows for correlation of intraoperative dynamic dosimetry to post-implant CT dosimetry.